Ultrasonic waves usually refer to waves of 16000 Hz or more, and since they enable non-destructive, harmless and essentially real-time examination of the inside of a body or material, they are applied in various fields such as detection of defects or diagnosis of disease. One of these applications is an ultrasonic diagnostic device used to generate images of the internal status within a subject by scanning the subject with ultrasonic waves and generating in image based on the reception signal generated from an ultrasonic wave (echo) reflected from the subject. This ultrasonic diagnostic device is compact and inexpensive in comparison with other medical imaging devices used in medical applications, is highly safe as a result of not causing radiation exposure associated with X-rays and the like, and offers various features such as display of blood flow using the Doppler effect. Consequently, ultrasonic diagnostic devices are widely used in fields such as circulatory organ (for example, diagnosis of the coronary artery of the heart), gastroenterology (for example, diagnosis of the stomach and intestines), internal medicine (for example, diagnosis of the liver, pancreas and spleen), urology (for example, diagnosis of the kidneys and urinary bladder) and obstetrics and gynecology. These ultrasonic diagnostic devices use an ultrasonic probe for transmitting and receiving ultrasonic waves (ultrasonic signals) to and from a subject. This ultrasonic probe is composed of one or more piezoelectric elements that generate ultrasonic waves (ultrasonic signals) by undergoing mechanical vibration based on a transmission electrical signal utilizing piezoelectric phenomena, and then generate a reception electrical signal by receiving a reflected ultrasonic wave (ultrasonic signal) generated due to mismatch of acoustic impedance within the subject.
As one aspect of this type of ultrasonic diagnostic device, research and development are being conducted on an ultrasonic diagnostic device that detects a second ultrasonic signal received from a subject based on a first ultrasonic signal transmitted to the subject by carrying out correlation processing on a reception signal obtained by receiving ultrasonic waves from the subject and a reference signal (template) for the purpose of receiving the second ultrasonic signal more accurately and/or observing deeper sites from the surface.
For example, in the ultrasonic diagnostic device disclosed in Patent Document 1, ultrasonic waves are diffused and transmitted to a prescribed three-dimensional region with an ultrasonic probe, a weighted matched filter, obtained by carrying out attenuation of ultrasonic waves corresponding to the distance from a reflection point to each ultrasonic transducer in a matched filter representing a correlation between a reflected wave from a prescribed reflection point contained in the prescribed three-dimensional region, is convoluted with a reception signal received by the ultrasonic probe to determine an image value at the prescribed reflection point, followed by determining volume data in the prescribed three-dimensional region by determining the image value of each reflection point. It is explained in paragraph [0020] of Patent Document 1 with respect to this matched filter that, “This matched filter is determined by the distance from each ultrasonic transducer of the ultrasonic probe to a prescribed reflection point and the speed of the propagating ultrasonic waves, and represents the correlation with a reflected wave from a prescribed reflection point received by each ultrasonic transducer. This matched filter is provided for each reflection point contained in the three-dimensional region.”
In addition, in the ultrasonic signal detection method using a matched filter disclosed in Patent Document 2, for example, a bottom echo signal waveform or flaw echo signal waveform is used a reference signal of a matched filter, a white noise waveform or grass echo signal waveform is used as a noise signal of the matched filter, and the coefficient signal of the matched filter is set based on the reference signal and noise signal. It is explained in paragraph [0048] of Patent Document 2 with respect to this matched filter that, “A bottom echo signal waveform or flaw echo signal waveform of an actual subject is used as a reference signal.”, and it is explained in paragraph [0054] of this document that, “A prediction signal from a system response is used as a reference signal”. Namely, a reference signal is obtained in Patent Document 2 by actually measuring an echo of a subject or by predicting from a system response.
However, in an ultrasonic diagnostic apparatus that detects a target signal, namely a second ultrasonic signal, by correlation processing in this manner, in the case the reference signal is not optimized, S/N ratio cannot be expected to be improved in comparison with the case of not carrying out correlation processing, and since there is also the occurrence of artifacts, optimization of the reference signal waveform is important.
Although the above-mentioned Patent Document 1 contains a description of a matched filter corresponding to a reference signal waveform, a specific method for forming a template of the matched filter is not disclosed, thereby making it difficult to obtain an optimum matched filter.
In addition, although a reference signal can be generated according to the description of the above-mentioned Patent Document 2, in the case of actual measurement, since the actual subject that generates the reference signal does not always coincide with a subject undergoing testing, it is difficult to obtain an optimum reference signal. In addition, in the case of predicting from a system response, since this is merely a prediction, the generated reference signal is not necessarily optimal.
Patent Document 1: Japanese Unexamined Patent Publication No. 2008-220652
Patent Document 2: Japanese Unexamined Patent Publication No. 2005-221321